Phys 2101
Gabriela González
•
Single particle:
•
Several particles:
v
m
p
=
d
p
dt
=
F
net
=
m
a
...
3
2
1
+
+
+
=
p
p
p
P
If F
net
=0, momentum is conserved
d
P
dt
=
M
a
com
=
F
ext
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Ricardo, mass 95 kg, and Carmelita, who is lighter, are enjoying
Lake Merced at dusk in a 35 kg canoe. When the canoe is at rest
in the placid water, they exchange seats, which are 3.0 m apart
and symmetrically located with respect to the canoe's center.
Ricardo notices that the canoe moved 40 cm relative to a
submerged log during the exchange and calculates Carmelita's
weight, which she has not told him. What is it?
Ricardo, mass 95 kg, and Carmelita, who is lighter, are enjoying Lake Merced at dusk in a 35 kg
canoe. When the canoe is at rest in the placid water, they exchange seats, which are 3.0 m apart
and symmetrically located with respect to the canoe's center. Ricardo notices that the canoe
moved 40 cm relative to a submerged log during the exchange and calculates Carmelita's
weight, which she has not told him. What is it?
R
C
B
X
com
R
C
B
X
com
40 cm
We know that the center of mass is 20cm from the
center of the boat, closer to Ricardo.
Choosing a coordinate system with the origin at the
center of the boat in the original position, the
coordinate of the center of mass is:
x
com
=
m
R
x
R
+
m
B
x
B
+
m
C
x
C
m
R
+
m
B
+
m
C
−
0.2
m
=
95
kg
×
(
−
1.5
m
)
+
30
kg
×
(0
m
)
+
m
C
(1.5
m
)
95
kg
+
30
kg
+
m
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 Fall '07
 GROUPTEST
 Physics, Mass, Momentum, Light

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